1. Field of the Invention
The present invention relates to an aluminum alloy clad material for heat exchangers exhibiting high strength and excellent corrosion resistance. More particularly, the present invention relates to an aluminum alloy clad material for heat exchangers exhibiting high strength and excellent corrosion resistance which is suitable as a material for a fluid passage (tube material) and a header plate material for automotive heat exchangers, such as a radiator or heater, joined by brazing using a fluoride-type flux or vacuum brazing, and is also suitable as a piping material connected to such heat exchangers.
2. Description of Background Art
As a tube material or header plate material for automotive heat exchangers such as a radiator or heater, a three-layered aluminum alloy clad material comprising a core material of an Al--Mn alloy such as a JIS3003 alloy which is clad with a brazing material of an Al--Si alloy on one side and a sacrificial anode material of an Al--Zn alloy or Al--Zn--Mg alloy clad on the other side has been used.
The Al--Si brazing material is clad to join a tube to a fin or header plate by brazing. As a brazing method, brazing in an inert gas atmosphere using a fluoride-type flux or vacuum brazing is employed. The sacrificial anode material constitutes the inner surface of the core material and exhibits a sacrificial anode effect by being in contact with a working fluid during use of a heat exchanger, thereby preventing pitting or crevice corrosion in the core material. The fin which is joined to the outer surface of the tube prevents the core material from being corroded by exerting the sacrificial anode effect.
In view of a reduced weight for automotive heat exchangers, the tube material has been provided with high strength by adding Cu to the core material, or by causing Mg and Si to coexist to produce an Mg.sub.2 Si compound in the core material and the sacrificial anode material to decrease the thickness of the tube material. Zn in the sacrificial anode material and Cu in the core material interdiffuse when heated for brazing and greatly affect the corrosion resistance. Because of this, the interdiffusion between Zn in the sacrificial anode material and Cu in the core material during heating for brazing has been taken into consideration to provide a clad material with excellent corrosion resistance and improved strength after brazing.
For example, a clad material in which the combination between the thickness of the sacrificial anode material layer and the Zn content is optimized (Japanese Patent No. 2,572,495), and a clad material exhibiting improved corrosion resistance in which the sacrificial anode material contains Zn and the core material contains less than 0.7% of Cu to set the potential difference between the core material layer and the sacrificial anode material layer at 30-120 mV (Japanese Patent Application Laid-open No. 023535/1994) have been proposed. A clad material provided with improved strength and corrosion resistance by adjusting the thickness of the sacrificial anode material layer to 46-70 .mu.m and adding 0.7-2.5% of Cu has also been proposed (Japanese Patent Application Laid-open No. 134574/1996).
A two- or three-layered aluminum alloy clad tube comprising a core material of an Al--Mn alloy such as a JIS3003 alloy clad with a sacrificial anode material of a Al--Zn alloy such as a JIS7072 alloy on either one side or both sides has been used for the passage connecting the automotive heat exchangers. The sacrificial anode material on the inner surface of the clad tube is in contact with a working fluid during use of a heat exchanger and exhibits the sacrificial anode effect, thereby preventing pitting or crevice corrosion in the core material. The sacrificial anode material on the outer surface prevents pitting or crevice corrosion in the core material, which occurs when used under severe conditions.
After brazing, the above conventional aluminum alloy clad materials for heat exchangers exhibit a potential gradient from the surface of the sacrificial anode material layer to the core material owing to interdiffusion between Zn in the sacrificial anode material and Cu in the core material. Corrosion spreads laterally in the direction of the plate width in such a gradient material having the potential gradient, whereby corrosion proceeds slowly to provide the material with excellent corrosion resistance.
However, the effective ranges for securing corrosion resistance in this method are limited because of limitations to the amount of Cu to be added to the core material, or requirement for providing the sacrificial anode material layer with a greater thickness in order to add a greater amount of Cu. If the thickness of the tube material is further reduced, corrosion resistance may be insufficient.